1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device, and more specifically to a method of manufacturing a semiconductor device in which flatness of the surface of an insulating film in a buried wiring formation area is ensured.
2. Description of the Related Art
In recent years, as semiconductor integrated circuits are made finer and are highly integrated, multilayering of wirings has been promoted. Under such circumstances, when a lower layer wiring is formed on a substrate interlayer insulating film formed on a semiconductor substrate and when an interlayer insulating film is deposited while covering the lower layer wiring, the surface of the interlayer insulating film formed on the lower layer wiring while covering the same is prevented from being flattened owing to a step between the ground interlayer insulating film and the lower layer wiring. Such unevenness of the interlayer insulating film formed on the lower layer wiring while covering the same prevents an upper layer wiring from being formed with high yield when the upper layer wiring is further formed on the interlayer insulating film. Accordingly, ensurance of flatness of the surface of the interlayer insulating film is important. For this, a buried wiring is formed in the interlayer insulating film to flatten the surface of the interlayer insulating film.
In the following, there will be described a method of forming a buried wiring with reference to FIGS. 9 and 10.
Firstly, as illustrated in FIG. 9(a), an interlayer insulating film 12 comprising a silicon oxide film is formed on a silicon substrate 11, over the entire area on which film a photoresist 13 is in turn applied to form a resist pattern corresponding to a configuration of wirings to be formed using a photolithography process, which pattern is used as a mask to form in the interlayer insulating film 12 trenches 14-1, 14-2, 14-3, . . . , 14-n with about 0.5 .mu.m depth, 0.3 to 10 .mu.m width, and about 0.5 .mu.m interval.
Thereafter, as illustrated in FIG. 9(b), the photoresist 13 is removed, and a tantalum (Ta) barrier layer 15 is deposited on side surfaces and on a bottom surface in the trenches 14-1 to 14-n and over the entire area on the interlayer insulating film 12, and further a copper seed layer 16 is formed over the entire area on the barrier layer 15. The deposition of the barrier layer 15 and the formation of the copper seed layer 16 are achieved respectively with a CVD method, a sputtering method, and the like, all well known. The barrier layer 15 is provided to prevent copper from diffusing into the silicon oxide film to produce a leakage current between the wirings or along a junction part in the silicon substrate.
As illustrated in FIG. 1(c), with a fountain plating method where the copper seed layer 16 is used as an electrode a copper plating layer 17 is deposited, which buries the trenches 14-1 to 14-n and has a substantially flat surface over a wide area on the interlayer insulating film 12.
As illustrated in FIG. 10, with chemical and mechanical polishing (CMP) the entire surface of the silicon substrate 11 on which the copper plated layer 17 is formed is polished until the interlayer insulating film 12 is exposed, to leave the copper plated layer 17 only in the trenches 14-1 to 14-n and hence form a buried wiring 18.
Referring now to FIG. 11, there will be described the fountain plating method in the process illustrated in FIG. 9(c). FIG. 11 is a schematical view exemplarily illustrating a fountain plating apparatus.
The fountain plating apparatus 21 includes as illustrated in the same figure a substantially cylindrical plating tank 23 for temporarily storing a plating solution 22 in which copper ion (Cu.sup.2 +) is dissolved, a cylindrical fountain cup 24 contained in the plating tank 23, a disk-shaped fixing plate 25 comprising an insulating material horizontally disposed slightly above the fountain cup 24, a plating solution fountaining fountain pipe 26 opened from a bottom surface of the fountain cap 24 upward at the center of the same, and a discharge pipe 27 for the plating solution 22 opened into the tank from a bottom surface of the plating tank 23. A pump and a plating solution tank are provided (not shown) outside the plating tank 23, and the plating solution 22 is introduced with use of the pump into the fountain cup 24 from the plating solution through the fountain pipe 26 and is returned from the plating tank 23 into the plating solution tank. The silicon substrate 11 is fixed to a lower surface 25a of a fixing plate 25 on which the copper plated layer 17 is to be deposited.
For forming the copper plated layer 17 on the silicon substrate 11 using the fountain plating apparatus 21 the silicon substrate 11 is first fixed to the lower surface 25a of the fixing plate 25, and then the fixing plate 25 to which the silicon substrate 11 is fixed is disposed horizontally at a predetermined position slightly above the plating solution 22, actuating the pump (not shown) to fountain the plating solution 22 from the fountain pipe 26. In this state, there is applied predetermined voltage where a fountain cup 24 side is set to be positive (+) and a silicon substrate 11 (copper seed layer 16) side is set to be negative (-) to conduct a current, and then a liquid level of the plating solution 22 is raised to fountain the plating solution 22 onto the surface of the silicon substrate 11 as indicated by an arrow whereby the copper plated layer 17 is deposited on the copper seed layer 16.
The plating solution 22 which has completed the deposition of the copper plated layer 17 overflows from an upper part of the fountain cup 24 to the side of the same. After the elapse of a predetermined time, the fountaining of the plating solution 22 is interrupted to lower the liquid level, and the fixing plate 25 is removed from the plating tank 23 and the silicon substrate 11 is removed from the silicon substrate 11. In such a manner, there is ensured the silicon substrate 11 where the copper plating tank 17 is deposited at a predetermined position. Although the aforementioned fountain plating apparatus 21 is an example where the fountain cup 24 is used as the positive electrode, another apparatus may be used in which a mesh electrode is provided in the fountain cup 24 to which positive voltage is applied, and in which the fountain cup 24 itself is not used as an electrode.
Although in the above description the fountain cup 24 side is set positive with the silicon substrate 11 side set negative, and predetermined voltage is applied thereacross to conduct a current, as described in Japanese Patent Laid-Open Application No. Sho57-71150, 1st to 7th lines on a left lower column, p.230, a plating speed at high current density is high in the fountain plating method so that no flat copper plated layer 17 is obtained when a fixed pattern current is conducted at all times.
To solve the problem there is known a plating solution (hereinafter, referred to a retarding agent) which is adsorbed at high current density portions on an exposed surface of the metal seed layer 16 or the copper plated layer 17 and into which there is added an additive that prevents copper from adhering to such portions (CubathM: trade name of ENTHONE OMI company, for example).
When a plating solution 22 containing such a retarding agent is used, the plating speed is slowed down automatically at high current density portions, so that a current having a unidirectional polarity at all times ensures a plated layer having a substantially flat surface. Accordingly, as illustrated in FIGS. 12(a) and (b), a negative DC current or negative DC pulsed current both having a unidirectional polarity is conducted to achieve fountain plating and hence ensure a copper plated layer 17 having a substantially flat surface.
There may also be a situation where a plating solution is used to which the foregoing retarding agent is not added (Microfab: trade name of EEJA company, for example). Since a plating speed at high current density portions is high, conduction of a current always having a unidirectional polarity brings about a problem that film thickness is nonuniform, and further as illustrated in FIG. 14 deposits adhere to portions at opposite sides in the vicinity of an opening where current density is high before the trenches 14-i are buried to produce voids 20 in the buried layer in the trench and hence shorten the life of electromigration.
To solve the problem, a DC pulsed current having a forwardly and backwardly alternating polarity as illustrated in FIG. 13 is conducted to achieve fountain plating and hence obtain a substantially, flat surface copper plated layer 17.
Such prior art as described above however suffers from a problem that when after the substantially flat surface copper plated layer 17 is obtained, polishing is applied to the surface and copper is left behind only in the trenches 14-1 to 14-n to form the buried wiring 18, erosion happens (a phenomenon where copper in a surface of a high wiring pattern density area is more polished and is indented) as illustrated in FIG. 10, and further when the copper plated layer 17 is buried in a wider trench 14-j to form the buried wiring 19 of a wider wiring pattern as illustrated in FIG. 15, dishing happens (a phenomenon where copper in a surface of a wider wiring pattern is more polished and indented) to limit the width of the wiring pattern and reduce the flexibility of a wiring design.
More specifically, in polishing with the CMP process, for leaving copper only in the trenches 14-1 to 14-n to form the buried wiring 18 while preventing the interlayer insulating film 12 from being reduced (within about 500 Angstrom), it is necessary to increase the polishing speed of the copper plated layer 17 than in the interlayer insulating film 12. For this, an acidic additive is added to an abrasive to oxidize copper into copper oxide whereby the copper is more rapidly polished than in the silicon oxide film.
Thus, when the interlayer insulating film 12 is exposed, the same is more removed at portions having a higher copper area ratio to result in the aforementioned erosion and dishing.